Field of the Invention: The present invention relates to a method and an apparatus for staining board and sheet material. More particularly, this invention relates to a method and apparatus for scrubbing a stain, particularly an aqueous latex stain, into the surface of a cellulosic sheet or board material.
Brief Description of the Prior Art: Staining compositions are used for protecting and coloring cellulosic, particularly wood surfaces. Until about 1970, oil-based stains had been used almost exclusively by both the commercial prestaining industry and by individual consumers. However, since 1970, aqueous latex stain has rapidly been supplanting the more commonly used oil-based stain, particularly among individual consumers. Aqueous latex stain has better durability and color stability than oil-based stain and cleans up easily with soap and water. However, aqueous latex stain has not been fully utilized by the commercial prestaining industry.
One of the problems with aqueous latex stain is that it does not adhere or penetrate the cellulosic material as quickly as oil-based stain. The commercial prestaining industry operates commercial staining equipment at fast line speeds. Since the aqueous latex stain does not penetrate the wood as quickly as oil-based stain, latex is not usable on most commercial prestaining equipment without extensive modifications.
In commercial prestaining, the cellulosic surface is first flooded with a staining composition followed by passing the flooded surface between nip rolls to force the stain into the surface and to remove excess which is collected and recirculated. The substrate is then passed beneath a rapidly rotating brush to remove any additional excess stain. When aqueous latex stain is employed with this type of equipment, very little of the stain anchors itself to the wood and as a result, only semi-transparent staining is obtained.
Although not completely understood, one of the reasons that latex stain does not adhere or penetrate as quickly to cellulosic substrates as oil-based stain is because of the different resin binders used in the two systems. In addition to pigment, oil-based stains contain linseed oil or alkyd resins or combination of these two as binders for the pigment. These resins penetrate cellulose quickly and secure the pigment firmly to the substrate. The resin and pigment is not readily brushed off the surface, thus allowing good solid hiding of the substrate. Latex resin molecules, on the other hand, are surrounded by a film of water which acts as a lubricant on the surface of the cellulosic substrate and can be easily swept away along with pigment and resin when moved under rapidly rotating brushes. This results only in semitransparent covering not solid hiding which is increasingly in demand.
The present invention provides a new method and machine for applying aqueous latex stain to cellulosic substrates. In the present invention, the aqueous latex stain is scrubbed into the cellulosic substrate. Thus, the present invention provides a method and apparatus which can be used by the commercial prestaining industry for applying aqueous latex stain to cellulosic substrates. The present invention offers many advantages over prior art equipment. In the present invention, only sufficient stain, which is required to get a desired degree of staining, is applied to the surface of the cellulosic substrate. There is no flooding of the surface with stain, or removal of excess stain, which was required by prior art techniques in the application of oil-based stains. Thus, the problems associated with these prior art techniques such as stain waste; recirculatory pump problems; long, difficult and costly clean ups and poor drying due to uneven coating are avoided.